1. Field of the Invention
The present invention relates to a DC-DC converter circuit for converting DC voltage to another DC voltage, a power supply selection circuit for selecting one of a plurality of power supplies, and an apparatus provided with such a DC-DC converter circuit.
2. Description of the Related Art
Many of portable type of electronic apparatuses such as a note personal computer and the like are so arranged that they operate from electric power obtained from a commercial power supply and a battery incorporated therein as well.
Usually, such an apparatus incorporates therein a circuit for changing over as to which source of electric power, the commercial power supply or the battery, is used to operate the apparatus (for example, Japanese Patent Laid Open Gazette Hei.9-182288, and Japanese Patent Laid Open Gazette Hei.9-308102). According to such type of circuit, when electric power obtained from the commercial power supply is supplied to the apparatus, this electric power takes precedence in use, and when the circuit detects that the supply of power from the commercial power supply stops, the supply of power changes to the supply of power from the battery. As another type of the power supply switching circuit, a circuit is arranged in such a manner that, in view of the fact that electric power obtained from the commercial power supply is generally higher in voltage than that from the battery, the supply of power selected is from the electric power of the highest voltage of the plurality of electric powers.
Incidentally, the voltage of a battery decreases as the battery discharges. Thus, an apparatus is provided with a DC-DC converter circuit for maintaining the voltage of electric power used in the apparatus.
FIG. 7 is a circuit diagram showing a first example of a linear regulator. The linear regulator is one type of a DC-DC converter circuit, and it is generally widely used.
A linear regulator section 10 is loaded on an LSI having an input terminal IN through which electric power of input voltage Vin is applied. The linear regulator section 10 converts the electric power of the input voltage Vin to electric power of output voltage Vout (Vin greater than Vout) lower than the input voltage Vin, and outputs electric power of the output voltage Vout through an output terminal OUT.
Between the input terminal IN and the output terminal OUT, an NPN transistor 11 for output voltage control is disposed, and between the input terminal IN and a base of the NPN transistor 11, a constant current source 12 is disposed. A current outputted from the constant current source 12 flows through the base of the NPN transistor 11 in the form of a base current thereof, and further flows through a collector of an additional NPN transistor 13 in the form of a collector current thereof. An emitter of the NPN transistor 13 is connected to a ground terminal GND, which is grounded. The output voltage Vout of the output terminal OUT is fed to a plus input terminal of a differential amplifier 16 in the form of a potential division by two resistances 14 and 15, while a reference voltage generated by a reference voltage source 17 is fed to a minus input terminal of the differential amplifier 16. An output terminal of the differential amplifier 16 is connected to a base of the NPN transistor 13.
In the event that the output voltage Vout of the output terminal OUT is biased with a voltage higher than a predetermined reference output voltage, the output voltage of the differential amplifier 16 increases, so that a collector current of the NPN transistor 13 increases. That is, of the current outputted from the constant current source 12, one used as the collector current of the NPN transistor 13 increases, and as a result, the base current of the NPN transistor 11 for output voltage control decreases and thereby the output voltage Vout of the output terminal OUT decreases.
Conversely, in the event that the output voltage Vout of the output terminal OUT is biased with a voltage lower than a predetermined reference output voltage, the output voltage of the differential amplifier 16 decreases, so that the collector current of the NPN transistor 13 also decreases. That is, the base current of the NPN transistor 11 increases and thereby the output voltage Vout of the output terminal OUT increases.
In this manner, the electric power of a constant output voltage Vout is outputted from the output terminal OUT.
FIG. 8 is a circuit diagram showing a second example of a linear regulator. The following description sets forth the differences from the first example of the linear regulator shown in FIG. 7, hereinafter.
A linear regulator 10xe2x80x2 shown in FIG. 8 is provided with a PNP transistor 18 for output voltage control, instead of the NPN transistor 11 for output voltage control in the linear regulator 10 shown in FIG. 7. As a result, the output voltage Vout of the output terminal OUT is fed to the minus input terminal of the differential amplifier 16 in form of a potential division by two resistances 14 and 15, while the reference voltage generated by the reference voltage source 17 is fed to the plus input terminal of the differential amplifier 16.
In the event that the output voltage Vout of the output terminal OUT is biased with a voltage higher than a predetermined reference output voltage, the output voltage of the differential amplifier 16 decreases, so that a collector current of the NPN transistor 13 also decreases. That is, of the current outputted from the constant current source 12, one used as the collector current of the NPN transistor 13 decreases, and as a result, the base current of the PNP transistor 18 decreases and thereby the output voltage Vout of the output terminal OUT decreases.
Conversely, in the event that the output voltage Vout of the output terminal OUT is biased with a voltage lower than a predetermined reference output voltage, the output voltage of the differential amplifier 16 increases, so that the collector current of the NPN transistor 13 also increases. That is, the base current of the PNP transistor 18 increases and thereby the output voltage Vout of the output terminal OUT increases.
In this manner, an electric power of a constant output voltage Vout is outputted from the output terminal OUT.
FIG. 9 is a circuit diagram showing a third example of a linear regulator.
A main difference from the second example of the linear regulator shown in FIG. 8 is that the PNP transistor 18 is replaced by P channel MOS transistor 19. With respect to circuit operation, it is the same as that of the second example shown in FIG. 8, and thus a redundant explanation will be omitted.
FIG. 10 is a circuit diagram showing an example of a switching regulator. The switching regulator 20 is also a type of DC-DC converter circuit, and it is generally widely used.
An electric power of voltage Vin is fed through an input terminal IN of the switching regulator, and an electric power of output voltage Vout (here dealing with a step-down type and thus Vin greater than Vout) is outputted from a second output terminal OUT 2, of first and second output terminals OUT 1 and OUT 2. Between the first and second output terminals OUT 1 and OUT 2, an outside coil 31 is connected. Between the second output terminals OUT 2 and the ground, an outside capacitor 32 is connected.
Elements of the switching regulator 20, except outside coil 31 and outside capacitance 32, are loaded on an LSI.
Between the input terminal IN and the output terminal OUT 1, P channel MOS transistor 21 is disposed. An output of a PWM comparator 26 is connected to a gate of the P channel MOS transistor 21. An output of a differential amplifier 24 and an output of a triangle wave generator 27 are fed to the PWM comparator 26. The PWM comparator 26 will be described later.
The voltage Vout of the second output terminal OUT2 is fed to a minus input terminal of the differential amplifier 24 in form of a potential division by two resistances 22 and 23, while a reference voltage generated by a reference voltage source 25 is fed to a plus input terminal of the differential amplifier 24. Between the first output terminal OUT 1 and a ground terminal GND which is grounded, a diode 28 is connected. A cathode of the diode 28 is connected to the first output terminal OUT 1, and an anode of the diode 28 is connected to the ground terminal GND.
The PWM comparator 26 compares an output voltage of the differential amplifier 24 with a triangle wave signal outputted from the triangle wave generator 27. When the output voltage of the differential amplifier 24 is lower in voltage than the triangle wave signal, the PWM comparator 26 generates a pulse signal of xe2x80x98Hxe2x80x99 level. When the output voltage of the differential amplifier 24 is higher in voltage than the triangle wave signal, the PWM comparator 26 generates a pulse signal of xe2x80x98Lxe2x80x99 level. Such a pulse signal is fed to the gate of the MOS transistor 21, so that the MOS transistor 21 turns on or off in accordance with the variation between the xe2x80x98Hxe2x80x99 level and the xe2x80x98Lxe2x80x99 level of the pulse signal. That is, the MOS transistor 21 switches the input voltage Vin at the same repetitive frequency as that of the triangle wave signal.
The diode 28, the coil 31 and the capacitor 32 smooth the input voltage Vin after the switching and generate the output voltage Vout.
When the output voltage Vout slightly exceeds a set up voltage, the output voltage of the differential amplifier 24 decreases, so that a pulse width (a pulse width of the xe2x80x98Lxe2x80x99 level) of the pulse signal generated by the PWM comparator 26 narrows slightly and thereby the output voltage Vout decreases. Conversely, when the output voltage Vout decreases, the output voltage of the differential amplifier 24 increases, so that a pulse width (a pulse width of the xe2x80x98Lxe2x80x99 level) of the pulse signal generated by the PWM comparator 26 expands and thereby the output voltage Vout increases. Thus, the switching regulator 20 controls the electric power of a constant voltage Vout to be outputted.
In an electronic apparatus, for example, a personal computer, there is frequently a case that a plurality of circuit units, operative with mutually different DC voltages, exist in the apparatus. Such an apparatus has a plurality of DC-DC converter circuits which output electric powers of individual voltages, respectively. A DC-DC converter circuit is associated with such disadvantages that a great deal of useless electric power is consumed for conversion of DC voltage, and as a result, the consumption of battery charge is hastened, and also this is associated with a temperature rise of the apparatus. For example, in case of the DC-DC converter circuit of the linear regulator scheme shown in FIGS. 7 to 9, for conversion from the input voltage of 16 volts into the output voltage of 3.3 volts, the conversion efficiency is 20%, and the remaining 80% is a power loss. Particularly, in an apparatus in which a plurality of mutually different DC voltages are used and a plurality of DC-DC converter circuits are needed in order to generate the plurality of mutually different DC voltages, it is a problem as to how the conversion efficiency is improved in the DC-DC converter circuits.
In view of the foregoing, it is an object of the present invention to provide a DC-DC converter circuit improved in conversion efficiency, a power supply selection circuit in which an existing DC-DC converter circuit is used to perform a voltage conversion improved in conversion efficiency, and an apparatus incorporated thereinto such a DC-DC converter circuit improved in conversion efficiency.
To achieve the above-mentioned objects, the present invention provides a first DC-DC converter circuit having a plurality of input terminals connected to a plurality of DC power supplies, respectively, and an output terminal. This DC-DC converter circuit has a power supply selection section for selecting the DC power supply of the lowest voltage on the condition that the voltage is not less than a predetermined voltage. This DC-DC convert or circuit also has a step-down type of regulator section for converting the voltage of the DC power supply selected by the power supply selection section into a predetermined voltage lower than the voltage of the DC power supply selected by the power supply selection section, and outputting the converted voltage through the output terminal.
As mentioned above, in case of the DC-DC converter circuit according to the linear regulator scheme, the conversion efficiency is 20% for a conversion of 16 V to 3.3 V. Conversely, in a case where a power supply of 5 V exists, the conversion efficiency is 66% for the same conversion. In this manner, when an output voltage is obtained from an input voltage which is close to the output voltage as much as possible, it is possible to greatly improve the conversion efficiency. This is applicable also to the switching regulator scheme as well as the linear regulator scheme.
The first DC-DC converter circuit according to the present invention utilizes this principle as mentioned above.
That is, the power supply selection section selects a DC power supply of the lowest voltage from among a plurality of DC power supplies, and transmits the selected DC power supply to the regulator section. However, in this case, in order to avoid such a situation that the lowest detected voltage is when no power supply is connected, or the connected power supply is not operative, so that the lowest voltage is 0 V, there is a requirement that the lowest voltage is not less than a predetermined voltage. The regulator section converts the voltage of the DC power supply thus selected to a DC voltage lower than the voltage of the selected DC power supply. Thus, it is possible to implement high efficiency voltage conversion wherein the optimum power supply is selected in accordance with the state of the power supplies.
To achieve the above-mentioned objects, the present invention provides a second DC-DC converter circuit having a first input terminal connected to a predetermined first DC power supply, a second input terminal connected to a predetermined second DC power supply of a voltage lower than that of the first DC power supply, and an output terminal. This DC-DC converter circuit has a power supply selection section for selecting the first DC power supply connected to the first input terminal and the second DC power supply connected to the second input terminal, the voltage of the second DC power supply being less than a predetermined voltage or is not less than the predetermined voltage, respectively. This DC-DC converter circuit also has a step-down type of regulator section for converting the voltage of the DC power supply selected by the power supply selection section into a predetermined voltage lower than the voltage of the DC power supply selected by the power supply selection section, and outputting the converted voltage through the output terminal.
In the event that it is decided that, as compared with the voltage of the first DC power supply entered through the first input terminal, the voltage of the second DC power supply entered through the second input terminal is lower, or it is arranged in such a manner as mentioned above on a connection basis, it is possible to simplify the power supply selection section in structure taking into account the idea of the first DC-DC converter circuit of the present invention.
In either of the first and second DC-DC converter circuits according to the present invention, it is acceptable that the regulator section have a linear regulator. In this case, it is preferable that the power supply selection section and the regulator section having the linear regulator are arranged in a chip of an integrated circuit. Or alternatively, it is preferable that the power supply selection circuit and portions of the regulator section having the linear regulator, except for an output voltage control transistor, are arranged in a chip of an integrated circuit.
In any of the first and second DC-DC converter circuits according to the present invention, it is acceptable that the regulator section have a switching regulator. In this case, it is preferable that the power supply selection section and portions of the regulator section having the switching regulator, except for a voltage smoothing circuit portion which is to be disposed outside, are arranged in a chip of an integrated circuit.
Arrangement in a chip of an integrated circuit makes possible a more stable operation, cost-reduction, and space saving.
To achieve the above-mentioned objects, there is provided a first power supply selection circuit having a plurality of input terminals connected to a plurality of DC power supplies; a power supply selection section for selecting a DC power supply of the lowest voltage, on the condition that the voltage is not less than a predetermined voltage, from among the plurality of DC power supplies; and an output terminal for outputting the voltage of the DC power supply selected by the power supply selection section.
To achieve the above-mentioned objects, there is provided a second power supply selection circuit having a first input terminal connected to a predetermined first DC power supply; a second input terminal connected to a predetermined second DC power supply of which the voltage is lower than the voltage of the first DC power supply; a power supply selection section for selecting the first DC power supply connected to the first input terminal and the second DC power supply connected to the second input terminal according to the voltage of the second DC power supply being less than a predetermined voltage or is not less than the predetermined voltage, respectively; and an output terminal for outputting the voltage of the DC power supply selected by the power supply selection section.
The first and second power supply selection circuits correspond to the power supply selection sections of the first and second DC-DC converter circuits, respectively. The DC-DC converter circuits corresponding to the regulator sections of the first and second DC-DC converter circuits are connected to the later stages of the first and second power supply selection circuits, respectively. This feature makes it possible to perform a highly efficient DC-DC conversion for the DC-DC converter circuits.
To achieve the above-mentioned objects, there is provided an apparatus operative upon receipt of an electric power having a step-down type of first DC-DC converter for converting a first DC voltage of a predetermined first DC power supply into a predetermined second DC voltage lower than the first DC voltage of the first DC power supply; a first operating circuit operative upon receipt of supply of an electric power of the second DC voltage obtained by the first DC-DC converter; a second DC-DC converter having a step-down type of regulator section for converting a received DC voltage into a predetermined third DC voltage lower than the received DC voltage, and a power supply selection section responsive to both the first DC voltage of the first DC power supply and an output of the first DC-DC converter for selectively transmitting to the regulator section the output of the first DC-DC converter and the first DC voltage of the first DC power supply according as the output of the first DC-DC converter is not less than a predetermined voltage or is less than the predetermined voltage, respectively; and a second operative circuit operative upon receipt of electric power supplied by the third DC voltage obtained by the second DC-DC converter.
The apparatus of the present invention as mentioned above is provided with two DC-DC converters of the first and second DC-DC converters. The second DC-DC converter, which outputs the lower DC voltage, is arranged with the first or second DC-DC converter circuit. This feature makes it possible to perform a DC-DC conversion excellent in efficiency, and also to implement a reduction of the consumed power and a suppression of temperature increase of the apparatus.
Generally, power supply systems are wired within apparatuses beforehand, and therefore the arrangement of the second DC-DC converter circuit of the present invention is generally used as the second. DC-DC converter. However, it is acceptable that the first DC-DC converter circuit of the present invention is used as the second DC-DC converter. At that time, the power supply selection section of the second DC-DC converter serves to block both the path for transmitting the output of the first DC-DC converter to the regulator section and the path for transmitting the voltage of the first DC power supply to the regulator section, when the first DC power supply is less than a predetermined voltage, in the event that the output of the first DC-DC converter is less than a predetermined voltage.